Endothelial cells also have surface receptors for bradykinin, tumor necrosis factor, components of the complement system, immunoglobulin crystallizable fragments, and adhesion molecules. In addition, vascular endothelial cells synthesize and secrete prostacyclin, endothelin, clotting factors, nitric oxide, prostaglandins, and cytokines. Vascular endothelial cells, however, lack 5-lipoxygenase and are not able to synthesize leukotrienes (smooth muscle constrictors). Compounds not metabolized by the pulmonary capillary bed include epinephrine, dopamine, histamine, isoproterenol, angiotensin II, and substance P.
Angiotensin-convertingenzyme(ACE)ispresentinsmallindentations(caveolae)onthesurfaceofpulmonaryendothelialcellsandcatalyzestheconversionofthephysiologicalinactiveangiotensinItotheactiveangiotensinII,apotentvasoconstrictor.Thisisamajormechanisminthebody’sabilitytosupplysystemiclevelsofangiotensinIIandthusinfluencebloodpressure(see
Chapter36).
ThetherapeuticuseofACEinhibitorsisimportantinthemanagementofpatientswithhighbloodpressure.
Thelungsplayakeyroleinthemetabolismofmanyprodrugs,whichareinactive,intoactivedrugs.Administrationofprodrugsimprovestheamountofactivedrugdeliveredtotheirtargetsinthebody.Inmanycases,inactiveprecursorprodrugsaredeliveredsystemicallyorlocally(throughinhalation)tothelungs,wheretheyareactivatedinsitu.Anexampleofthisisbeclomethasonedipropionate(Qvar,BeconaseAQ),amedicationinhaledbypatientswithasthma,whichisactivatedbyesteraseswithinthelungtotheactiveform17-beclamethasonemonopropionate.
1.
The respiratory system has developed unique structural (mucociliary transport system) and immunological (mucosal immune system) features to cope with the constant environmental exposure to foreign substances; these features limit or inhibit inflammation.
2.
The three components of the mucociliary transport system are the sol phase (periciliary fluid), the gel phase (mucus), and cilia.
3.
The depth of the periciliary fluid layer is maintained by the balance between Cl− secretion and Na+ absorption and is essential to normal ciliary beating.
4.
Mucus is a complex macromolecule composed of glycoproteins, proteins, electrolytes, and water. It has low viscosity and high elastic mechanical properties.
5.
Goblet cells, Clara cells, and the mucous and serous cells residing in the tracheobronchial glands produce mucus.
6.
Particle deposition in the lung is dependent on their size, density, and shape; the distance traveled; airflow speed; and relative humidity. The major mechanisms for particle deposition are impaction (particles larger than 10 µm, in nasal passages, and particles 2 to 10 µm, in the nasopharynx, trachea, and bronchi), sedimentation (particles 0.2 to 2 µm in size, in distal airways), interception (particles with elongated shape, in the lower airways), and brownian movement (particles smaller than 0.2 µm, in the alveoli).
7.
The respiratory system is part of the mucosal immune system, which is composed of the intestinal (GALT), the respiratory (BALT), and urinary tract systems. These systems do not contain true lymph nodes with afferent and efferent lymph flow; they are composed mainly of nonencapsulated lymph nodules without true lymphatic drainage.
8.